1. Field of the Invention
The present invention relates to the improvement of a fixing device used in an image forming device such as a printer or a copying machine using an electrophotographic system, an electrostatic recording system etc. and the improvement of an image forming device using the fixing device.
2. Description of the Related Art
Conventionally, as such a kind of the image forming device, there is know one in which toner images of respective color components formed on a photosensitive member etc. are primary-transferred sequentially on an intermediate transfer member and multi-color toner images on the intermediate transfer member are collectively transferred on a sheet of paper by a secondary transfer device (collectively transfer device).
In recent years, such a color image forming device has been strongly required to be high speed, be miniaturized and save energy. In particular, a heat fixer, which most consumes energy in the image forming process, has been required to be applied with an energy saving technique (low power technique) aimed to reduce environment load.
In accordance with such a demand, there is disclosed a technique in which heat of a sheet of paper after the fixation and transported from a fixing device is directly or indirectly transmitted to a sheet of paper before the fixation to pre-heat the sheet of paper prior to the fixing process, thereby to reduce heat energy required in the fixing process performed after the pre-heating and increase a fixing speed without increasing an electric power required for heating (see JP-A-8-38412 (embodiment, FIGS. 2 to 15) and JP-A-2000-33803 (embodiment, FIG. 1), for example).
However, in such a technique, the configuration to make a sheet of paper after the fixation directly contact a sheet of paper before the fixation can be applied only to a continuous sheet of paper (roll sheet of paper). That is, since such a technique cannot be applied only to a cut sheet of paper, there arises a problem that the general purpose property of a sheet of paper to be applied is limited.
Further, as shown in FIG. 10, there is proposed a method in which, at the time of fixing a toner image transferred from a photosensitive member 101 on a sheet of paper 102 by a fixing device 103, a heat pipe 106 is provided between an upstream side portion 104 and a downstream side portion 105 of the fixing device 103 thereby to transmit heat of the sheet of paper 102 after the fixation at the downstream side portion 105 to the upstream side portion 104 (see JP-A-8-38412, for example).
In such a configuration having a heat pipe with a block or the configuration in which heat of a sheet of paper after the fixation is transmitted to a sheet of paper before the fixation via a heat transmission unit such as a heat transmission block, a heat transmission plate or a combination of a heat transmission roller and a belt, a cut sheet of paper can be applied and so the general purpose property of a sheet of paper can be secured. However, in order to make a heat quantity steady at the heat transmission unit to obtain sufficient heat absorption/heat transmission effect, it is required that the temperature of the heat transmission unit is repeatedly increased through heat transmission from sheets of paper and approaches the temperature of a sheet of paper just after the fixation. That is, in such a configuration, since it takes much time to increase the temperature of the heat transmission unit, there arises a problem that heating energy for the fixing procedure cannot be reduced immediately.
Particularly, in such a case, a middle or small sized image forming device which frequently performs printing or copying operation of several ten sheets of paper or less can hardly attain the aforesaid effect.
Further, in a method of applying auxiliary heat to the heat transmission unit in order to attain the heat absorption/heat transmission effect immediately (increase the rising speed of the effect), much energy is consumed and so this method is not a good way for saving energy.
Further, as shown in FIG. 11, there is proposed a method which is arranged in the following manner (see JP-A-8-38412, for example). That is, a belt 201 and a paper transport belt 202 are disposed in an opposite manner, a heater 203 is provided at the inside of the belt 201 and press-contact plates 204 are provided at the both sides (upstream and downstream sides) of the heater 203 to constitute a press contact state between the belt 201 and the paper transport belt 202 thereby to transport a paper 205 between the belt 201 and the paper transport belt 202. In this configuration, further a heat transmission belt 206 that moves in an opposite direction to the moving direction of the paper transport belt 202 is provided at the inside of the paper transport belt 202 to transmit absorbed heat at the downstream side of the heater 203 toward the upstream side thereby to preheat the paper 205.
However, in such a configuration, the transport belt to transport a sheet of paper does not serve as a heat transmission medium and a heat transmission belt is provided separately from the transport belt, whereby the configuration is complicated and the heat transmission efficiency is degraded.
As described above, each of the aforesaid conventional techniques merely transmits heat of a sheet of paper after the fixation indirectly to a sheet of paper before the fixation via a heat absorption transmission unit.